Two concentric spheres (hollow) are shown in the diagram. What charge should be given to the smaller sphere, so that, the potential at point $P$, outside the sphere, is zero?

(Radius of inner sphere $=r$, Radius of outer sphere $=R$ and charge on outer sphere $=Q$) 

Four capacitors with capacitances $C_1=1 \mathrm{\mu F},C_2=1.5 \mathrm{\mu F}$, $C_3=2.5 \mathrm{\mu F}$ and $C_4=0.5 \mathrm{\mu F}$ are connected as shown and are connected to a $30$ volt source. The potential difference between points $a$ and $b$ is:

A charge Q is uniformly distributed over a long rod AB of length L as shown in the figure. The electric potential at the point O lying at a distance L from the end A is :

A total charge $Q$ is uniformly distributed over a long rod $\mathrm{AB}$ of length $L$ as shown in the figure below. The electric potential at the point $\mathrm{O}$ lying at a distance $\frac{L}{2}$ from the end $\mathrm{A}$ is